1. Field of the Invention
This invention relates generally to power cycles and control systems for power cycles. More particularly, this invention concerns a variable pressure power cycle and control system which is capable of adjusting pressure on the heating phase of the cycle in response to a variable heat source fluid inlet temperature.
2. Description of the Prior Art
In the past, a typical approach to designing the parameters for a power cycle has been to first select the cycle and then a working fluid for the cycle. The power cycle is selected based on its geometric compatibility with a heat source and a heat sink. The working fluid is selected based on the physical constraints imposed by the mechanics of the process.
The heat source and heat sink define a thermodynamic envelope within which the power cycle must operate. That is, because of the inefficiencies of heat transfer, a temperature difference will exist between the working fluid and heat source fluid in a heat exchanger and also between the working fluid and the heat sink.
The objective of the process designer is to devise a thermodynamic cycle that will encompass the largest possible area within the thermodynamic envelope defined by the heat source and the heat sink in order to maximize work output. For a constant heat source, i.e., a condensing vapor heat source, the rectangular shaped Carnot cycle describes a theoretically most efficient means of power generation. In practice, the conventional subcritical Rankine cycle, which has a boiling working fluid, most efficiently utilizes the available heat from a condensing vapor heat source.
When, however, the heat source is a liquid phase heat source in which the temperature of the heat source fluid drops through the heating phase of the cycle, the simple Rankine cycle is inefficient. Examples of liquid phase heat sources include liquid dominated geopressure-geothermal resources and processed waste liquids from processes such as petrochemical, nuclear and the like. The increase in energy demand makes recovery of the available energy from these resources an economically feasible proposition.
Many techniques have been used to alter the shapes of simple power cycles to approximate a series of Carnot cycles, either horizontally or vertically assembled, to match the declining temperature thermodynamic envelope of the heat source. For example, double boiling cycles have been utilized to more closely approximate the thermodynamic envelope. A major drawback of this system is that the cycle requires complex equipment including a two-phase heat exchanger, a mist extracter and a complex control system.
Recent studies have shown that a supercritical Rankine cycle is superior to the subcritical cycle in liquid phase heat source applications. Although a supercritical heat exchanger may have a larger surface area than a subcritical exchanger, it is simpler in mechanical design than the two-phase heat exchanger and mist extracter required for subcritical cycles. More pump work is generally required by the supercritical process due to the higher working pressures required for operation which in turn require more structural material in the piping and heat exchanger. In the past, the increased capital requirements for heavier hardware have led to economic compromises in cycle design based upon cheap fuel in favor of lower pressure processes. Further, higher heat transfer coefficients for two phase systems imply a reduction in heat exchanger surface area and hence a reduction in construction material requirements. These traditional arguments in favor of two phase systems have been weakened by a major shift in the cost relationship between energy and capital equipment brought about by increased energy demand.
Traditionally, power plant cycles are designed based on a fixed maximum pressure for the cycle. This maximum pressure occurs during the heating phase of the cycle and is maintained by varying the flow rate of the working fluid through the system.
These fixed pressure power cycle systems work well when the heat source maintains a constant inlet temperature and flow rate over time. However, when the inlet temperature of the heat source varies (e.g., a geopressure-geothermal heat source) or if it is desirable to move the power plant from one heat source to another heat source having a different inlet temperature or flow rate (e.g., moving the power plant from one geothermal well to another), a fixed pressure power cycle possesses an inherent shortcoming--inflexibility. Specifically, a fixed pressure cycle defines a thermodynamic envelope that is incapable of adjusting to substantially fill the changing thermodynamic envelope that is defined by a changing heat source or different heat sources. For variable heat source applications, it is therefore desirable to provide a power cycle in which the maximum pressure developed during the heating phase of the cycle can be varied, thereby varying the temperature over the heating phase to more closely match the temperature of the heat source for maximum heating efficiency. Further, it is desirable that a control system be provided to automatically adjust the heating phase pressure of the cycle in response to a changing heat source.